The research program is directed toward a longitudinal appraisal to temperament in infant twins. The determinants of temperament appear to be inherent in origin, and there are wide individual differences among infants in the basic patterns of reactivity displayed in the early months of life. Newborn twins offer a powerful resource for assessing the similarities and differences in temperament among genetically related pairs, and for examining the continuities in the temperament over age. Twins also furnish a unique opportunity to observe infant-mother reciprocity in cases where the two infants may differ markedly in temperament, and thus evoke quite different reactions from the mother. The principal aims of the program are (a) to perform a comprehensive appraisal of temperament in twins from birth to three years, based on direct observations in standardized test situations; (b) to coordinate these data with the extensive at-home observations reported by the mother, (c) to implement a psychophysiology program for coordination with the behavioral measures of temperament; (d) to assess the joint contribution of genetic and environmental factors to temperament; and (e) to videotape the facial expressions and behavior of the twins, for comparison with each other. The reference test situation is centered around the administration of the Bayley Scales, with a separate observer making detailed behavioral ratings throughout the test. Other ratings are obtained upon the initial arrival of the twins and separation from the mother, during a warm-up period spent with the examiner, during a play period following the test, and while being undressed and restrained for physical measurement. The scoring is designed to yield a composite profile of temperament that will display individual differences between infants, and permit a test of similarities within twin pairs. Prior work on this project has focused on early mental development, and several studies are scheduled for completion with the current sample of twins and siblings.